The present invention relates to anti-icing ef rotating nose bullets of gas turbine engines.
Heating of rotating nose bullets presents particular problems. For example, electrical heaters cannot be incorporated, as they could be with static nose bullets, without the use of slip rings, or other such complications, for carrying the heater current across the interface between static and rotating parts of the engine. Other methods have been tried, for example, ducting hot air from other parts of the engine to the nose bullet and allowing it to impinge thereon, or to form a film of hot air over the internal surface thereof. Once again this has had the disadvantages that the hot air either has been allowed to escape from the nose bullet into the air stream entering the engine compressor where it distorts both the pressure and temperature profiles of the air, or has had to be ducted away to another part of the engine to avoid heating the compressor discs. This adds complexity to the engine design.
Some shapes of nose bullet e.g. pointed, or ogival, have been found to be satisfactory without specific anti-icing provisions, but the problem still remains for blunt nose bullets, which are preferred in some engine installations because of their shorter length.
We have now found that it is possible to achieve satisfactory anti-icing of the whole of the spinning nose bullet, provided that a relatively small area adjacent the axis of rotation is maintained free of ice.
British Patent No. 2046843 discloses a very successful way of anti-icing a rotating nose bullet. The method described in British Patent No. 2046843 makes use of the temperature difference which occurs between different axial positions along the engine. In this way, energy can be made available to produce a rotating "thermosyphon" effect using the nose bullet as the cooler and using a region downstream of the nose bullet as the heat input. Hot gas is caused to flow along the centre of a hollow rotating tube or drive shaft of the engine towards the nose bullet. The denser cooled air is centrifuged against the wall of the bore of the shaft and flows back along the shaft to exit from the rear of the shaft.
The above mentioned British Patent also discloses an embodiment which uses a hollow rotating shaft which is closed off at the front end by a nose bullet assembly and open at the rear. It is explained in the patent that although the open ended tube is not per se a thermosyphon it works in a similar manner to a thermosyphon in that the centrifugal forces on the different densities of gas pump the hot gases down the centre of the tube to heat the nose bullet and pump the cooled gases rearwards against the shaft wall.
Whilst the anti-icing system of British Patent No. 2046843 works extremely well, it has been found that in some engine arrangements there is insufficient flow of hot air down the centre of the rotating tube. This is particularly so where the source of hot gases is a swirling mass of gases from the vicinity of the turbine components. It has been found that in engine arrangements of the latter mentioned type where the rotor causes the inflowing air to swirl, the flow of gases along the centre of the shaft is seriously impaired and may be stopped completely.